全新java初学者实践教程10(java SE5.0版)
-likj#Z jdk5的集合类
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[|(N_[E|6 YKH\rN6X pWK(z[D 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
x[h^[oF0 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
ts\5uiB<% [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
}1Gv)l7 eg(6^:z?f [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
gW>uR3Ca4 gQ'zW [font="Times](1) Set
#_6I w`0 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
Q=AavKn# :S<f?*
}: ([font="Times]2)[font="Times]List
gl\\+VyU 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
/?@3.3sl_ pGJ>O/% uE%r/:!k4$ [font="Times] ([font="Times]3)[font="Times]Map
i~I%D%; [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
}001K bCo7*<I4 fZ0M%f [font="Times] ([font="Times]4)[font="Times]Queue
=G7m)! [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
,l!Ta" [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
_FH`pv B8f8w)m `|{-+m 实践:[font="Times] Set举例
oW ::hB [font="Times] import java.util.*;
"e.jZcN* [font="Times]public class SetExample {
7
n8"/0kc: [font="Times] public static void main(String[] args) {
fI&t] [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
U>]$a71 [font="Times] set.add("one");
_I@9HC 4 [font="Times] set.add("second");
}=< [font="Times] set.add("3rd");
YC++&Nk [font="Times] set.add(new Integer(4));
Z/k:~%|E [font="Times] set.add(new Float( 5.0F ));
kW;+|qs^ [font="Times] set.add("second");
#Y*X<L [font="Times] set.add(new Integer(4));
llcb~ [font="Times] System.out.println(set);
,{M^-3C [font="Times] }}
)'l:K.F j[`j9mM8 [font="Times]List举例:
n^Hm;BiE# [font="Times] import java.util.*;
6 :b!F [font="Times]public class ListExample {
TE3lK(f [font="Times] public static void main(String[] args) {
d,+Hd2o^X [font="Times] List list = new ArrayList();
B2>H_dmQ [font="Times] list.add("one");
;LcZ`1 [font="Times] list.add("second");
U'H$`$Ov [font="Times] list.add("3rd");
U{2BVqM [font="Times] list.add(new Integer(4));
J!c)s!`w [font="Times] list.add(new Float( 5.0F ));
$xzAv{ [font="Times] list.add("second");
\k .{-nh [font="Times] list.add(new Integer(4));
B<5R [font="Times] System.out.println(list);
X{5vXT\/y [font="Times] }}
S\:P-&dC ZP@
$Q%up >0/i[k-dk [font="Times]Map举例
q!.byrod [font="Times]import java.util.Map;
)
i;1*jK [font="Times]import java.util.HashMap;
~IYUuWF( [font="Times]import java.util.Iterator;
- Ajo9H [font="Times]import java.io.FileReader;
] eotc2?u [font="Times]
jyZ (RB [font="Times]public class MapExample {
aS{|uE] [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
l3Xfc2~ 2 [font="Times] Map word_count_map = new HashMap();
Sc\*W0m [font="Times] FileReader reader = new FileReader(args[0]);
u(@$a4z [font="Times] Iterator words = new WordStreamIterator(reader);
'))0Lh
l [font="Times]
pmuvg6@h [font="Times] while ( words.hasNext() ) {
aOlT;h [font="Times] String word = (String) words.next();
n&$j0k [font="Times] String word_lowercase = word.toLowerCase();
@i2E\} [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
CDsSrKhx [font="Times]if ( frequency == null ) {
J l(&!?j [font="Times] frequency = new Integer(1);
LInz<bc<( [font="Times] } else {
YWe{juXSw [font="Times] int value = frequency.intValue();
mk;&yh [font="Times] frequency = new Integer(value + 1);}
4w*Skl=F} [font="Times] word_count_map.put(word_lowercase, frequency);
fz|cnU [font="Times] }
IHB}`e| [font="Times] System.out.println(word_count_map);
XW[j!`nlk [font="Times] }}
`F-/QX[: Oxm>c[R LhA*F[6$M [font="Times]Queue举例:
(up~[ [font="Times]import java.io.IOException;
i+A3~w5c [font="Times]import java.io.PrintStream;
{#ZlM [font="Times]import java.util.LinkedList;
*:Y%HAy* [font="Times]import java.util.Queue;
RSfQNc9Z [font="Times]
@Xt*Snd [font="Times]public class QueueTester {
,|3MG",@@h [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
6/5YjO|a [font="Times] public QueueTester() {q = new LinkedList();}
,c;Kzp>e [font="Times]public void testFIFO(PrintStream out) throws IOException {
H3z:ZTI [font="Times] q.add("First");
{x|[p_? [font="Times] q.add("Second");
8m-U){r!U^ [font="Times] q.add("Third");
\HqNAE2T [font="Times] Object o;
t)~"4]{*}D [font="Times] while ((o = q.poll()) != null) {
@@R7p [font="Times] out.println(o);}}
,BH@j%Jmy [font="Times] public static void main(String[] args) {
z6U\axO6 [font="Times] QueueTester tester = new QueueTester();
IbT=8l,Li [font="Times] try { tester.testFIFO(System.out);
s]HOGJJz [font="Times] } catch (IOException e) {
P@Hs`= [font="Times] e.printStackTrace(); } }}
"i
nd$Z`c V[RF</2T {:Orn%Q 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
( Z619w 总结:
Yrb{ByO& 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
C].iCxn 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。